Dispensing nozzles are well known for dispensing fluids supplied directly or indirectly from storage containers to a substrate. They are used widely in industry for many applications. One particular application is the use of nozzles to dispense adhesives as sealants in the provision of gaskets in the automotive industry. In robotic applications the nozzle is incorporated onto a programmed automated arm which then moves the nozzle about a substrate in a predetermined path. (See e.g. U.S. Pat. No. 5,215,034 Ronsheim.) FIG. 1 of the drawings accompanying the present patent application shows an example of an anaerobic gasketing application utilising one such known dispensing nozzle 100 having a circular aperture dispensing adhesive 110 onto a substrate 120 from a height of about 5 mm. When dispensing in such an arrangement there is a tendency for the adhesive to curl up and stick to the outside of the end portion 130 of the nozzle 100, thereby interrupting the flow of adhesive from the end portion of the nozzle to the substrate and resulting in breaks 140 in the applied adhesive on the substrate. It is possible to lessen this effect by reducing the height of the nozzle end portion above the substrate, which for this particular application is found to have an optimum height of about 2 mm above the substrate.
In the provision of liquid gasketing materials it is common to use anaerobic adhesives. These adhesives are so called because they do not cure in the presence of air. As such the adhesives are typically supplied in mechanically sealed permeable containers with air contained therein. The nature of the constituents of the adhesive, and the fact that it is the presence of air that inhibits curing, means that it is essential for the adhesive to contact air during storage and prior to application. Otherwise, the shelf life of the adhesive becomes compromised.
Despite treatment to eliminate larger air bubbles, it is not uncommon for these products to have bubbles contained therein, along with dissolved air. The presence of the air bubbles in the adhesive tends to cause breaks in the flow from the container and in the adhesive bead created. After the passage of a bubble out of the dispensing tip, normal adhesive is dispensed again, which re-establishes the continuity of the adhesive bead. The surface area of the exit port of the dispensing nozzle and the height of the nozzle from the substrate affect the quality of the bead being laid down. If a break occurs and the nozzle is at a large distance away from the substrate then the length of the break as seen on the substrate will be large.
It is believed that air bubbles tend to vent at the wall surface of the nozzle and if such bubbles are successfully vented at the exit port no break in the bead will occur. However, a circular exit port has only a limited wall surface, corresponding to the circumference of the circle, and it is believed that some bubbles tend to become trapped at the centre of the product flowing out of the exit port and do not reach the wall surface for venting.
The minimum bead size that should be applied to a substrate is in many cases a matter of judgement. If an excessively small bead is specified small bubbles in the adhesive may cause breaks in the applied bead. A more substantial bead would envelop a small bubble so that a good quality application of the product to the substrate without a break could be achieved despite the presence of the bubble. There is, therefore, a balance between applying an unduly large amount of product, which would be expensive, and applying a bead so small that it can not be reliably dispensed without blockages. In general users of automated dispensing systems want smaller beads and therefore smaller nozzles than have been used heretofore.
Many gasketing products are resin based which gives them particular properties when being dispensed. They generally have a high viscosity and a yield point, and at low shear stresses behave like solids. When these products are being dispensed and are between a nozzle and the substrate they can behave like a string, rather than as a flow of liquid. If the velocity of the product relative to the nozzle is less than the robot speed, the product will be under tension. A bubble in the adhesive will tend to cause the bead to break, even when the original bubble itself is small enough to be incorporated into the bead. It is more desirable to push the product from the nozzle onto the substrate, rather than pull it from the nozzle onto the substrate. The effect of any bubbles is minimised the faster the product flows from the nozzle, which suggests that the ratio of product velocity to the robot velocity is important; current understanding recommending a minimum ratio of 1 and preferably of at least 2. The velocity ratio may also be expressed as an area ratio as follows:       R    v    =                    V        P                    V        R              =                  A        B                    A        N            where:    RV=The velocity ratio    VP=The velocity of the product relative to the nozzle    VR=The velocity of the robot    AN=The cross-sectional area of the nozzle    AB=The cross-sectional area of the bead.
The cross sectional area of the bead can be calculated from the dispensing flow rate and the robot speed as follows:       A    B    =      w                  V        R            ⁢      t      ⁢                          ⁢      ρ      where:    w=the weight of a quantity of dispensed product    t=the dispense time    ρ=the product density.
In the accompanying drawings, FIG. 1a is a graph of nozzle diameter (mm) against bead cross section (mm2) for two velocity ratios:                               R          v                =                                            A              B                                      A              N                                =          1                                and                                    R          v                =                                            A              B                                      A              N                                =          2                    
Dotted lines across the graph correspond to circular nozzles having diameters of 0.84 mm (AN=0.55 mm2), 1.19 mm (AN=1.11 mm2), 1.36 mm (AN=1.45 mm2) and 1.54 mm (AN=1.86 mm2). The best area on the chart is below the lower curve (Rv>2). A compromise is the area between the curves (1<Rv<2). A nozzle should be chosen from the chart so that it intersects the lower curved line at a bead cross-section value that is less than the desired bead size. It is evident using this basis that the diameter of the nozzle outlet has a significant effect on the quality of the dispensed bead of adhesive, with smaller nozzle diameters achieving better quality of dispensed product of small cross-sectional areas.
Although it is preferable to use smaller diameter nozzles so as to achieve improved dispensed bead quality, there are limitations in achievable dispensed bead cross-section relative to nozzle outlet diameter. There are problems in using smaller nozzle diameters in that the nozzle diameters are also governed by the particle size of the filler or partly cured polymer in the product. Desirably, the products are filtered to exclude particles having a size greater than 300 microns, and it is recommended that the nozzle diameter should be at least three times the filter size. With conventional circular cross-sectional area nozzles it has been demonstrated that in order to achieve good quality bead cross-sectional areas of less than approximately 1.2 mm2 it is necessary to reduce the nozzle outlet diameter to, or below, approximately 0.84 mm, which is less than the recommended three times filter size. Using such small nozzle diameters introduces problems into the dispensing system as the nozzles are prone to blocking or clogging if any large particles are present in the material being dispensed.
There therefore exists a need to provide a dispensing nozzle that simulates a small circular cross sectional area, thereby providing good quality dispensed beading, yet is not prone to the effects of blockages.
Various forms of nozzle are known for use in fields other than automated dispensing systems (e.g. manual dispensing devices). U.S. Pat. No. 3,884,396 Gordon et al. refers to nozzles known in the field of caulking guns and also the field of cake or candy decorating devices, including a nozzle which has a star-shaped orifice. As pointed out in the '396 patent, such nozzles are made of rigid material where the shape of the orifice remains unchanged over a wide range of extruding pressure, since the purpose of the nozzle is to form an extrusion having a definite cross-sectional shape, i.e. corresponding to that of the orifice. The '396 patent itself is directed to a dispenser for salad dressing or mayonnaise. The manually-operated apparatus has an extrusion nozzle comprising a core made of flexible and resilient material, with a circular orifice portion lying in a plane transverse to the axis of the core and truncating the core near its apex, and a plurality of slots formed in the core and extending in a converging relation from points near the base of the core into the circular orifice portion, the material of the core lying between the slots comprising a plurality of flexible resilient fingers. These pliable petal-like blades open as rapidly as required under varying degrees of pressure to define a temporary orifice of larger area through which contents can only emerge at low velocity. The circular orifice is defined by the tips of the fingers, which are separated by the slots, i.e. the nozzle is not continuous in the plane of the orifice. The fingers flex under normal dispensing conditions to give a larger orifice and this will allow for an increase in volumetric flow without a significant increase in velocity. This is contrary to what is required for a good quality bead and the nozzle of the '396 patent would not be suitable for automated dispensing.
German Gebrauchsmuster G 94 11 980.5 Ritter describes a closure for the month-piece of a cartridge, particularly for printing pigments. The closure is in the form of a membrane extending transversely across the mouth, with radical slits which define tongues in the membrane. As shown in FIG. 2B, the membrane has a very small space at the tips of the fingers and effectively has no dispensing opening in the rest condition. When pressure is applied to the cartridge, in normal dispensing conditions, the tongues flex axially outwards (see FIG. 3A). This also would be unsuitable for forming a good quality bead in automated dispensing.
U.S. Pat. No. 4,981,629 Cook describes a tool for applying caulking manually. The dispensing opening has a generally trapezoidal appearance. It is clear from the description of the use of the tool that it is made of rigid material. The '629 patent does not provide any teaching relevant to automated dispenser systems.
German Offenlegungschrift DE 37 23 100 A1 shows apparatus for dispensing a filler around a fixing. The spray tube has a funnel-shaped broadened rim with notches or indentations. Despite the state-of-the-technology it would be desirable to provide a dispensing nozzle for dispensing fluids in bead form in an automated dispensing system.